St. Pierre played five games with the Blackhawks last season and spent the most time with Chicago's AHL affiliate Rockford. He led the team in scoring, finishing with 21 goals and 67 assists in 69 games.
Bruins general manager Peter Chiarelli said St. Pierre will provide depth at the center position.
Pelletier played in six games last season with Boston. He scored 71 goals in 206 regular season games with Providence of the AHL between 2005-08.
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